This invention relates to a torque measuring device, and particularly, to such a device employing dual load cell assemblies which provide high accuracy over a large range of measurement.
Precision torque measuring devices are employed in numerous mechanical devices. One such application where highly accurate torque measurements are needed is in dynamometer devices. Dynamometers are often used for measurement of the output horsepower of a prime mover or measurement of losses which occur in transmitting torque through machine components. Typically, the machine to be tested on a dynamometer is mounted to a torque balance frame which is not restrained torsionally. Rotation of the torque balance frame is resisted by an extending torque arm which is coupled to a load cell device. Forces measured by the load cell are related to the torque loads generated by the test article. While such devices generally operate satisfactorily, there is a continuing need to provide such systems with greater accuracy over a broad range of applied torques. Conventional load cell designs possess a direct trade-off in terms of their useful measuring range versus accuracy. This limitation is inherent since the resistance strain gauges or other strain measuring devices within a load cell are capable of accurate measurements over a relatively narrow range and can be damaged by excess strain.
In view of the foregoing, it is desirable to provide an improved torque measuring device featuring high accuracy over a large range. In accordance with this invention, such improvements are provided through the use of a pair of load cell assemblies, one of which being employed to measure low torque levels, and a second being employed to measure high torque levels. A compliant element is provided which couples a dynamometer torque arm to the low range torque measuring load cell assembly and lost motion means are provided for the high range load cell assembly. With this arrangement, only the low range load cell assembly is loaded at low torque levels. Higher applied torque loads cause forces to be applied to both load cell assemblies such that the total torque is calculated by summing the readings from both load cells.
The above arrangement results in substantially eliminating any lack of homogeneousity between the high torque range and the low torque range because the readings of the two cells are always added. Thus, for readings in the low torque range, the cell for the high torque range reads a zero or constant low reading for which the torque measuring device can compensate. Similarly, for readings in the high torque range, the cell for the low torque range reads a substantially constant, maximum or saturated value. Therefore, because the readings of the two cells are always added to one another, there is no lack of homogeneousity between the two torque ranges, and a smooth, substantially continuous total measuring range is obtained.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.